Mercury Content in Anguillicola crassus ( Nematoda ) and its Host Anguilla anguilla

Palíková M., V. Baru‰: Mercury Content in Anguillicola crassus (Nematoda) and its Host Anguilla anguilla. Acta Vet. Brno 2003, 72: 289-294. Mercury content in parasite-host system (Anguillicola crassus and Anguilla anguilla) was studied in the Koryãany Reservoir (Czech Republic). The swimbladder nematode contained the lowest mercury levels (0.050 0.183mg kg-1 wet weight), (p<0.05), compared to muscles (0.162 0.827) and the liver (0.175 1.430) of the fish definitive host. The mercury concentrations in the definitive host were higher in the liver, but not significantly different from muscle concentrations. Regression analysis indicated a positive ratio between mercury concentrations in the liver and the total body length of the fish. In eels from the reservoir under study the mercury content in fish muscles exceeded the hygienic limit in one fish specimen only. Muscle, liver, parasite-host system There is a developing body of literature analysing different amounts of heavy metals in fish and their organs and tissues (survey in Jezierska and Witesca 2001). The same tendency is manifested in comparative quantitative studies of various fish-parasite host systems (survey in Sures et al. 1999; Tenora et al. 2000; Turãeková et al. 2002). The parasite-host system of the nematode Anguillicola crassus Kuwahara, Niimi et Itagaki, 1974 and its definitive host the European eel, Anguilla anguilla L. was studied relatively frequently. Sures et al. (1994) analysed the lead content in this nematode and its host from nature only, while Zimmermann et al. (1999) did it experimentally. More heavy metal burdens (lead, cadmium, chromium and nickel) in these parasite-host systems were reported by Tenora et al. (1999). The content of selected microelements in males and females of A. crassus were studied by Baru‰ et al. (1999). Data on concentrations of mercury in this parasite-host system were still lacking. In this study, therefore, we deal with the first assessment of mercury concentrations in eels and its specific parasite A. crassus. Materials and Methods The eels used in this study were caught on March 13, 2001 in the Koryãany reservoir in South Moravia (Czech Republic) by electrofishing. The Koryãany reservoir has 35.15 hectares and 20 metres in maximum depth. The description of the locality and occurrence of A. crassus was published in Pal íková and Navrát i l (2001) or in Navrát i l et al. (1999). All fish specimens were placed in aerated aquaria filled with water from the collection site, and immediately transferred to the laboratory. In the laboratory, specimens were killed after three days and examined for metazoan parasites using standard procedures. The examination was aimed at the infection by the swimbladder nematode A. crassus, in particular. Total length (Tl in mm), weight (w in g) and infection intensity by A. crassus were measured in individual fish specimens (Table 1). Samples of muscles, liver of all eels (n = 10) and collective samples of nematodes from infected eels (n = 7) were stored at a temperature of -18 °C. Samples were digested using HNO3 for the determination of mercury in closed system. Mercury was determined using ColdVapour Atomic Absorption Spectrometry (CV-AAS). The reproducibility precision as a relative standard deviation, limit of detection and limit of determination are in Table 2. Parallel duplicate analysis of each sample ACTA VET. BRNO 2003, 72: 289–294 Address for correspondence: MVDr. M. Palíková, PhD. Department of Biology and Wildlife Diseases University of Veterinary and Pharmaceutical Sciences Palackého 1-3, 612 42 Brno, Czech Republic Phone: +420 541 562 821 E-mail: palikovam@vfu.cz http://www.vfu.cz/acta-vet/actavet.htm

was made.Minimum two blank samples in each round (eight to nine samples) were used.Each second round contained the sample of reference material.Each third round contained the matrix spike sample (•ucman et al. 2001).
The results were statistically analysed by the Student's t-test and c 2 -test using the STATplus software (Matou‰ková et al. 1992).

Results
The mercury concentrations in eel tissue samples and swimbladder nematode samples are summarized in Table 3.
It is evident that the A. crassus biomass contained the lowest mercury level compared to both eel tissues.It is well expressed by the ratio R = concentration Hg (parasite)/concentration Hg (host tissues), according to Sures et al. (1999), which were in all cases below 1.Values of the ratio R (parasite/muscle) varied from 0.18 to 0.78 (mean 0.39); while the ratio R (parasite/liver) varied from 0.09 to 0.77 (mean 0.33).Differences of concentrations of the mercury in the parasite biomass and host tissues were significant (p < 0.05).Comparing the eel tissues (muscle vs. liver), the mean value of the Hg concentration was higher in the liver, but in individual samples it was six times higher in the liver and four times in the muscle.The difference of Hg concentrations in tissues of eels (muscle and liver) was not significant.
Linear regression was used to evaluate relations between concentrations of mercury and total length of fish specimens (Tl).This resulted in a lower level of significance only in the ratio of Hg (liver) versus Tl (fish) ), R 2 = 0.501; whereas for concentrations of Hg in the muscle and the parasite vs. Tl (fish) the values of the regression coefficient were not significant (Fig 1).

Discussion
From the data presented here, one may conclude that the swimbladder nematode A. crassus accumulates less mercury than the tissues of its final host eel.Thus, it seems possible that this specialised hematophagous parasite is able to excrete the mercury in detoxication processes.The same trends and results were published by Sures et al. (1994) in relation to lead concentrations in A. crassus (mean 0.02 ± 0.01; range 0.01 -0.03 in µg g - 1 wet matter), and in liver of naturally infected (n = 8) eels (0.18 ± 0.05; range 0.12 -0.28) from the river Weser (Germany).The size (age groups) of adult worms did not differ in this aspect.The results in this parasite-host system were confirmed experimentally by Zimmermann et al. (1999).
The four heavy metal concentrations (lead, cadmium, chromium and nickel) in this parasite-host system were analysed by Tenora et al. (1999).Regarding the methodical differences (concentrations of heavy metals were expressed in µg g -1 of 100% of dry matter), the authors found higher concentrations in males and females of the A. crassus than in the muscle samples of eels for lead (mean values 1.41 ± 0.02 in males; 0.41 ± 0.03 in females; 0.18 ± 0.03 in muscle), for cadmium (0.74 ± 0.01 in males; 0.20 ± 0.01 in females; 0.09 ± 0.02 in muscle), for chromium (1.05 ± 0.03 in males; 0.35 ± 0.03 in females; 0.08 ± 0.02 in muscle) and for nickel (1.21 ± 0.03 in males; 0.39 ± 0.02 in females; 0.08 ± 0.02 in muscle).It is remarkable that in the biomass of A. crassus males (which in the adult stage are always of smaller dimensions and consume evidently less host blood during their life than females do) there were found higher heavy metal concentrations.Sexually determined differences in concentrations of selected microelements (iron, zinc, manganese, copper and cobalt) were also found in A. crassus biomass by Baru‰ et al. (1999).The survey of results, however, indicates unambiguously that the accumulation and concentration values of heavy metals in A. crassus are in comparison to other fish parasite species (acanthocephalans and tapeworms, in particular) considerably lower.Values of the ratio (R) vary according to Tenora et al. (1999) for males of A. crassus vs. eel muscle in the range of 7.8 -15.1, for females of A. crassus vs. muscle between 2.3 -4.9, only.For the mercury, according to our results the efficacy of accumulation and the values of concentrations in A. crassus are comparably low as for the lead (Sures et al. 1994).According to Tenora et al. (2000) the only species of the nematode, Philometra ovata Zeder, 1803, infecting the body cavity of cyprinid fishes is known to accumulate more heavy metals than the muscle of the host, i.e., the bream (Abramis brama L.).The respective values for lead, chromium and cadmium in the parasite were 106, 43.5 and 119 times higher than in fish muscle samples.
Low efficacy of mercury accumulation (and also of other heavy metals) that we found in A. crassus enables to exclude in this parasite-host system the influence of the parasite on decreasing the heavy metal concentrations in body tissues of infected fish specimens.The influence of infection by the tapeworm Proteocephalus percae Müller, 1780 in the perch (Perca fluviatilis L.) on decreasing of concentrations of cadmium, copper, zinc and arsenic was described by Turãeková andHanzelová (1999) andTurãeková et al. (2002); for lead, nickel and chromium concentrations in the barbel (Barbus barbus L.) infected by tapeworm Bathybothrium rectangulum Bloch, 1782 it was also mentioned by Baru‰ et al. (2001).
In this way, the eel may serve as a fish species for effective bio-indication in dependence of the region (e.g.Pieters and Geuke 1994).
The data on mercury concentrations in eels from various localities in the Czech Republic were presented by Studnicka et al. (1974); Hejtmánek et al. (1975); Svobodová and Hejtmánek (1976);PeÀáz et al. (1980) and Svobodová et al. (1988Svobodová et al. ( , 1993)).In eels from rivers the mercury concentrations ranged from 0.045 to 0.661 mg kg -1 wet matter in muscle samples, and from 0.341 to 1.365 in liver tissue.In eels from the water supply reservoirs (Îelivka reservoir, in particular) mercury concentrations ranged from 0.506 to 2.40 in muscle, and from 0.286 to 5.76 in liver.Mason (1987) recorded the mercury concentration in the range 0.09 -0.66 mg kg -1 in eel flesh from East Anglia Sites, Richards and Dulley (1983) in the range of 0.07 -0.68 in eels from tidal Thames.According to Barak and Mason (1990) in the liver and muscle of eels the concentrations of the mercury varied between 0.02 -0.38 and 0.09 -0.83, respectively, in fish from the Rivers Brett and Chelmer in eastern England.Mercury levels in eel flesh and liver tissue in the study site were similar to those reported from this fish species in the Czech and the British waters.According to Barak and Mason (1990), Svobodová et al. (1999) and Goldstein et al. (1996) in highly polluted sites the level of mercury in liver tissues and gonads increased significantly compared to muscle tissue, particularly in predator fish species.Concentrations of mercury found at the site under study indicated rather lower mercury load.This was the case because even with higher mean mercury concentration in the liver as compared to the muscle the difference was not significant.The mercury content in the liver regressed on the Tl (age) of eels only, but not significantly.This result was probably influenced by temporal variability, small number of fish in collection, and also habitat preference during the season.
According to Czech assessment values, the content of mercury in muscle tissues higher than 0.6 mg kg -1 in predatory fish species is considered as significantly exceeding hygienic limits.Predators (eels in this study) from the Koryãany reservoir exceeded the limit only in one fish specimen examined, while in all others the concentrations are evidently lower.

Table 1
Total length (Tl), weight (w) and intensity of infection by A. crassus in eels (A.anguilla) analysed for Hg concentration; np = no parasites in the swim bladder

Table 3
Mean mercury concentrations (mg kg -1 wet weight) in tissues of A. anguilla and in the parasite A. crassus; np = no parasite in swim bladder (mean value ± SD of repeating measurement) Fig 1. Concentrations of Hg in the host A. anguilla (muscle, liver) and in the parasite A. crassus